Investments - Investments in Continuous Unrealized Loss Position for More Than Twelve Months (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 81,183	$ 24,196
Unrealized Losses	(114)	(20)
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	3,996	0
Unrealized Losses	(4)	0
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	77,187	24,196
Unrealized Losses	$ (110)	$ (20)

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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